The following meanings for the abbreviations used in this specification apply:    3GPP 3rd generation partnership project    DeNB Donor evolved Node B    DL Downlink    DSP Digital signal processor    ECGI E-UTRAN Cell Global Identifier    eNode-B evolved Node B (also referred to as eNB)    FDD Frequency-division duplex    ICIC Inter cell interference coordination    IM Interference management    IP Internet protocol    LOS Line of sight    LTE Long term evolution    LTE-A LTE-Advanced    MBSFN Multicast/broadcast single frequency network    OFDMA Orthogonal frequency-division multiple access    O&M Operation and Maintenance    NLOS Non line of sight    PCI Physical-layer cell identity    PRB Physical resource block    RAN Radio access network    RN Relay node    RPW Resource partitioning window    RSRP Reference signal received power    RSRQ reference signal received quality    SON Self organizing network    TDM Time-division multiplexing    TTI Transmission time interval    UE User equipment    UL Uplink    UMTS Universal mobile telecommunications system    VoIP Voice over IP
Embodiments of the present invention relate to mobile radio communications with focus on LTE-A. In particular, embodiments of the invention target relaying as a feature of LTE-A. Relaying is a promising technology to extend cell coverage, in particular to provision high data rates in high shadowing environments (e.g. indoor coverage), and to enhance cell capacity with a low cost for LTE-A systems. FIG. 1 shows the typical LTE radio access scenario including relay nodes (RNs). There are basically three different links in such a deployment:                direct link (the connection between a donor evolved Node B (DeNB) and user equipment (UE))        backhaul link or relay link (connection between a DeNB and an RN) and        access link (connection between an RN and a UE)        
An interface between the DeNB and the RN is defined as Un interface, and an interface between the RN and the UE is defined as Uu interface, as the interface between eNB/DeNB and the UE.
There are many types of relays which might be applicable to different scenarios. Type 1 relay node (RN) is being specified during the work item in LTE Release 10 (as described, e.g., in TR 36.814 v9.0.0 (2010-03), Further Advancements for E-UTRA, Physical Layer Aspects). Type 1 relay is an in-band relay, which will use the same frequency band for relay link and access link, and which controls its own cell, and a unique physical-layer cell identity (PCI) is provided for each of the cells. The same RRM mechanisms and protocol stacks are available and from a UE perspective there is no difference between the cells controlled by a relay and the cells controlled by a “normal” eNB. Additionally, the cells controlled by the relay should also support LTE Release 8 UEs, because a mandatory backward compatibility is required. Moreover, for Type 1 relay, access link and relay link transmissions are time multiplexed, which means the relay cannot communicate with UEs it serves (RN-UEs) and DeNB simultaneously. There is a defined time frame consisting of several transmission time intervals (TTIs), referred in the following as resource partitioning window, where one subset of the TTIs is used for relay links (also referred to as Un sub-frame configuration) and the complementary part for the access links (FIG. 2). Note that in FIG. 2 the TTIs assigned for the relay links are shown to be consecutive as an example and that they can also be non-consecutive. All of the available resources can be used for the direct links, i.e. they share the resources with the relay links (the user scheduler at the DeNB decides to schedule relay or direct links on any particular physical resource block (PRB), i.e. smallest frequency-time resource element of the orthogonal frequency-division multiple access (OFDMA) system) and use the same resources as the access links using for instance interference coordination (ICIC) means.
Due to this time multiplexing feature of the relay link and access link for Type 1 relay, a new type of interference—which does not exist in the original system without relays—will be introduced by relays, namely relay-to-relay interference (RN-to-RN interference) among different relays. This kind of interference will exist for both the downlink (DL) and the uplink (UL), and mainly arises when the transmission of a relay fully or partially overlap the reception of another relay. In the following section, this interference problem in relay systems is illustrated in detail.
In relay deployments, a new type of interference may occur: the RN-to-RN interference. It does not exist in other heterogeneous networks like Femtocells, Pico eNB deployments, etc. and therefore, this type of interference and its impact have not been properly investigated yet; however, solutions to coordinate it are now becoming necessary.
FIG. 3 presents example scenarios where RN-to-RN interference is experienced. In UL (dashed lines), RN-to-RN interference occurs when RN “B”-to-eNB “B” communication interferes with the victim UE “A”-to-RN “A” communication producing an interfering signal. In case of no power control (UEs and RNs transmit at maximum available power) interfering signal could be even much higher than the wanted signal due to the difference in antenna gains, transmission powers, etc. between UE “A” and RN “B”. This is mostly because RN “B” has typically a higher transmit power than the UE and also has an antenna with higher gain, and furthermore its antenna is at higher altitude and will therefore more likely have a line-of-sight (LOS) connection to the victim RN's antenna. On the other hand, RN-to-RN interference could get even worse if power control (e.g. LTE Release 8 compliant power control) is applied. Here, the worst case scenario occurs when the aggressor RN has non-line-of-sight (NLOS) connection to its DeNB (i.e. it is transmitting with higher power to cover the strong attenuation due to the NLOS link) and has a LOS link with the victim RN. Further, in DL (solid lines), RN-to-RN interference occurs when RN “A”-to-UE “A” communication interferes with the victim eNB “B”-to-RN “B” communication producing an interfering signal at the victim RN “B”. Considering the close deployments of RNs and good propagation conditions on the inter-RN channel, such interference can be significant as compared to the received signal on the RN at the macrocell edge.
From what preceded, it is clear that RN-to-RN interference will significantly degrade the access and relay link qualities in UL and DL, respectively. This, for example, could be the case in the following example scenarios:
Neighbouring RNs may be controlled by different independent DeNBs which are using different resource partitioning (different set of TTIs for access and relay links), i.e. no coordination (particularly Un sub-frame coordination) among DeNBs. Thus, it may be the case that there are contemporary transmission on access link for one RN and the backhauling (i.e. transmission on relay link) on the neighbouring RN. FIGS. 4A and 4B presents an example of such a case, where different TTIs are utilized in different cells to serve the access and relay links. In particular, FIG. 4A shows the case without coordination.
Note that even if coordination exists, different loads in the cells might lead to different sub-frame configurations. This could even be the case within the same cell, where different RNs have different configurations to achieve better utilization of both relay and access link resources (e.g. needed for capacity enhancement in hotspots).
In case where DeNBs in the network are coordinated and apply the same resource partitioning, i.e. same set of TTIs are configured in both cells to serve the access or relay links, it might still be the case that RN-to-RN interference takes place. This is mainly due to the lack of inter-DeNB time synchronization in frequency division duplex (FDD) systems, which might create a time offset and hence results in a partial overlapping between relay link and access link transmissions in different cells. FIG. 4B presents a visualization of this scenario, i.e., the case with coordination.
Hence, there is an urge to tackle this problem.